Polarization reference for beam flying

ABSTRACT

This invention relates to airborne antenna systems for the reception of signals from stations transmitting bearing information as displacement from a course-line or fixed beam such as VOR or runway localizer stations. In addition to the usual antenna for receiving the course displacement signals, an antenna element is provided having an output signal which is responsive to the attitude of the airplane as referred to the polarization-plane or the radio field. This polarization-referenced signal is modulated for identification prior to being applied, together with the regular antenna signal, to the input terminal of a standard navigation receiver. At the receiver output, the modulation is recovered and applied to a phase-detector to provide an attitude signal containing information, necessary for stable beam flight, which would otherwise have to be obtained from gyro-horizon and directional gyro instruments.

United States Patent 1191 Brady et al.

[ 1 POLARIZATION REFERENCE FOR BEAM FLYING [76] Inventors: Frank B.Brady, 5322 Carvel Road, Washington, DC. 20016; Chester B. Watts,Jr.,6505 Pinecrest Court, Fairfax County, Va. 22003 [22] Filed: Jan. 9, 1970[21] Appl. No.: 1,742

[52] US. Cl. 343/107, 343/100 PE, 343/108 R Primary Examiner-Benjamin A.Borchelt Assistant Examiner-Richard E. Berger Attorney-Wilfred G.Caldwell NAV GATlO/V 1 June 26, 1973 [57] ABSTRACT This inventionrelates to airborne antenna systems for the reception of signals fromstations transmitting bearing information as displacement from acourse-line or fixed beam such as VCR or runway localizer stations.

In addition to the usual antenna for receiving the course displacementsignals, an antenna element is provided having an output signal which isresponsive to the attitude of the airplane as referred to thepolarizati0n-plane of the radio field. This polarizationreferencedsignal is modulated for identification prior to being applied,- togetherwith the regular antenna signal, to the input terminal of a standardnavigation receiver. At the receiver output, the modulation is recoveredand applied to a phasedetector to provide an attitude signal containinginformation, necessary for stable beam flight, which would otherwisehave to be obtained from gyro-horizon and directional gyro instruments.

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POLARIZATION REFERENCE FOR BEAM FLYING BACKGROUND-SUMMARY The techniqueof flying beam systems by a combination of raw displacement and courseheading has long been recognized as more difficult than flying computedcommand signals.

This difficulty is apparent in flying VOR, TACAN, and particularly ILSlocalizer with a simple unaided dis placement instrument and a magneticheading reference. The pilot must intercept the course, turn to courseheading and then bracket the course in a series of small course changesto correct for cross-wind and compass errors. His corrected heading,once established, will change if wind shear is present. This procedurecan be very demanding, particularly when other factors such as rough airare encountered.

Excellent instrumentation and flight control systems, incorporatinggyros, have been developed which relieve the pilot of this mentalcomputation and ease the workload involved in this process. The SperryZero Reader, using command signals only, and the Collins Flight Directorsystem with a combination of command and situation (displacement andheading have successfully overcome the difficulties mentioned.Unfortunately, their high cost and complexity put them out of reach ofmost owners and pilots of small aircraft.

In the airborne reception of localizer and VCR sig nals by the usualnavigation receiver, not all of the potentially useful information isextracted from the signal. The radio fields which are radiated by VORground stations, and by localizers near the course-line, arehorizontally polarized with high purity; vertical component is down 26decibels or more. Detection of the plane of polarization of the radiosignal can, thus, provide a good indication of the aircraftroll-attitude. In addition, sensing the azimuthal angle of arrival ofthe signal can provide an indication of aircraft heading relative to thetransmitting station. These two components of the aircraft attitude areexactly the information needed, combined with the basic coursedisplacement, to provide the command or what-to-do type of indicationwhich results in stable beam flight even in the hands of a pilot withvery little training.

It is an object of this invention to provide a signal responsive to theaircraft attitude derived from the normal radio fields received at theaircraft from VCR and ILS localizer stations. This may be done by meansof an auxiliary antenna element, a modification of the regular airbornenavigation antenna, or a re-designed combination antenna. This signal,modulated for identification, passes through the standard navigationreceiver along with the regular modulations which indicate displacementfrom the course-line. At the receiver output, the attitude signalmodulation is converted to a bi-polar d. c. signal compatible with theregular deviation indicator signal, which is ordinarily plus or minus150 millivolts, full scale. Adding these two signals produces anull-bank command signal; the pilot banks to keep the command signalnulled.

This simple derivation of steering or command signals makes it possibleto include the advantages of advanced instrumentation in classes ofaircraft in which gyros with electrical pickoff and special displayswould 'be regarded as too expensive.

LIST OF FIGURES FIG. 1 (a) and (b) show views of the tail and forwardportions of an airplane structure indicating several locations andantenna types suitable for providing navigation andpolarization-referenced attitude signals.

FIG. 2 (a) shows a modulation and signal-combining block diagram withseparate navigational and polarization reference antenna elements.

FIG. 2 (b) is the same as FIG. 2 (a) but for combination dual-modenavigational and polarization reference antenna elements.

FIG. 3 is a block diagram showing interconnections and major componentsrequired to complete an operating polarization-referenced null-bankcommand systern.

FIG. 4 defines the geometry used in computing the polarization referenceantenna pick-up as a function of the bank angle, relative heading, andforward tilt.

FIG. 5 provides an intuitive visualization of the flightpath followed bya pilot using a null-bank command meter to equate the polarizationreference signal to the course displacement signal.

DESCRIPTION For airborne reception of the localizer and VCR navigationalsignals, an antenna that has been generally successful is the balancedV-type mounted on the tail atop the vertical stabilizer. See, forexample, FIG. 21 of Hurley, Anderson and Keary, CAA VI-IF Omnirange,Proc. I. R. 12., vol. 39, pp 1506-1520, December, 1951. In FIG. 1 (a),rods 2 and 4 comprise the elements of such a balanced V-type antenna,the angle between the rods being more or less. This antenna receiveshorizontally polarized signals from front and rear about equally, and isdown roughly 6 to 8 decibels at the sides.

For the purposes of this invention, the auxiliary monopole antenna, rod6 is added to the balanced V- type antenna. Rod 6 lies in the verticalfore-and-aft plane of symmetry of the airframe, but may be tiltedforward, by an angle G, from the vertical. From considerations ofsymmetry, it is apparent that rod 6 will pick up little or no signalwhen the airplane is headed, with wings level, directly toward ahorizontally polarized transmitting station. Changes in thepitch-attitude do not affect this condition, but changes in either rollor yaw-attitudes will in general result in some signal in rod 6.

The connections to rods 2, 4, and 6 are shown in FIG. 2 (a). Rods 2 and4, comprising the horizontal V-type antenna, are generally connected tosome kind of balun 8 for providing phase relation between the rods. Thebalun output cable 10, would ordinarily simply connect to the navigationreceiver input, but in this case, the signal goes first through theisolation hybrid l2, thence to the receiver via cable 14. Thepolarization reference signal from rod 6 is fed into the balancedmodulator 16, which is preferably a four-diode ring or some other simpletype, supplied, via cable 18, with the modulating signal from asinusoidal source of some suitable frequency, such as 1.7 kc/s, withinthe receiver band-width. The sideband signal thus produced is carried bythe adjustable-length cable 20 to hybrid 12, combining there with theregular navigational signal, thence to the receiver via cable 14. It isnecessary to chose a length for cable 20 such that the 1.7 kc/ssidebands, when added in hybrid 12 to the carrier of the navigationalsignal, are properly phased for amplitude modulation. This can beaccomplished, for example experimentally, by varying the length of cableto obtain a maximum of 1.7 kc/s voltage at the receiver output.

A popular alternate form of navigation antenna is the V-type mounted ona short mast over the forward cabin roof, such as antenna 22, FIG. I(b). Sometimes the mast portion of such an assembly is arranged to befed separately as a vertical radiator for communications, as for exampleNARCO Catalog No. VCNA-2. In such a case, the mast can, instead, be usedas the polarizationreference element, substituting for rod 6 in FIG. 2(a). This has the advantage of accomplishing the purpose of thisinvention with a pre-existing antenna structure.

Another suitable antenna scheme is to use just two elements such as rods24 and 26, FIG. 1 (b), inclined in mirror symmetry about the airframevertical centerplane. The rods may be separated by an angle such as 80,more or less, the angle bisector being, on the center-plane, tiltedforward from the vertical by the angle G. FIG. 2 (b) shows how the tworods 24 and 26 may be made to function in two modes with the additionalhybrid 28. The difference port of this hybrid is associated with a 180phase relation of the elements, serving the same purpose as balun 8 inFIG. 2 (a). In the difference mode, rods 24 and 26, therefore, performas a balanced V-type antenna, similar to rods 2 and 4, except for thetilt in the vertical plane. The tilt angle G, in this case, ispreferably not less than 45, in order not to sacrifice too muchhorizontal component. The regular navigational signal thus flows fromthe difference port through cable 30 to the isolation hybrid l2, thenceto the receiver via cable 14 as before. In the sum mode, rods 24 and 26have a zero phase relation, as though they were simply connected inparallel with each other. It is well known that two thin radiatorsconnected in parallel behave much like a single fat one. The sum modethus serves effectively as a replacement for the auxiliary monopole, rod6. The polarization reference signal then is carried by cable 32 fromthe hybrid sum port to the balanced modulator 16, to be processed asbefore.

Yet another suitable form of antenna 34, FIG. 1 (a), utilizes a pair ofcavity-backed slots, one on either side of the vertical stabilizer, theeffective plane of electric field of the slots being inclined fromvertical by the angle G. Such slot antennas can serve the purpose ofthis invention by substitution for the rods 24 and 26 in the dual-modecircuit of FIG. 2 (b).

FIG. 3 shows a block diagram of the main components and interconnectionsneeded to complete a polarization reference system. Terminal 36 connectsto the standard navigation receiver 38 antenna input, receiving thesignal from cable 14 in FIG. 2 (a) or (b). Line 40 carries thereceiver's bi-polar d. c. output which drives the pilots coursedeviation indicator needle. This signal, identified here as E1, is fedalso, via line 42, to the filter-adder network 44. The receiver audiooutput line 46, feeds headset 48, and also the 1.7 kc/s band-pass filter50. The output of filter 50, representing the polarization-referencedattitude signal, is then fed to phase detector 52 for conversion to abipolar d. 0. signal identified here as E2, and carried via line 54 tothe filter-adder network 44. The 1.7 kc/s gource 56 supplies, viaterminal 58, the modulating signal for the balanced modulator 16 in FIG.2 (a) or (b). It also supplies, via line 60, the phase referencerequired by the phase detector 52. The filter-adder net' work 44 will ingeneral comprise resistance and capacitance elements as desired forsmoothing or otherwise time-modifying the signals E1 and E2, beforecombining them to form the algebraic difference (E1 E2), fed via line 62to null-bank command meter 64.

The behavior of signal E1 with position of the airplane is well knownand is assumed to be in accordance with published descriptions of thesystem being flown, and is relatively independent of the attitude of theairplane; The behavior of the attitude signal E2, as generated in thisinvention, may be computed approximately from a consideration of theorientation of the auxiliary monopole, such as rod 6, with respect tothe electric field vector of the radio signal radiated by the station.Signal E2 should be relatively independent of the displacement from thecourse-line.

In the argument which follows, it is assumed that we have a simpletheoretical monopole, in which the axis of the radiation patterncoincides with the physical axis of the rod. This is not really truebecause there are currents on the airframe skin under the monopole whichgenerally do not have axial symmetry with the rod. Nevertheless, theassumption will serve well enough for a rough calculation if we takecare to locate the monopole on the airframe where it can be reasonablyperpendicular to the local surface and also have a clear view forward tothe transmitting station.

FIG. 4 shows a set of rectangular coordinates, centered on the airplaneat the base of the monopole. The Z-axis always points to the zenith, andthe X-axis is always in the direction of the transmitting station, withthe radiated electric field vector thus coinciding with the Y-axis. Theforegoing holds true while the maneuvering of the airplane changes themonopole orientation; the projection of the monopole upon the Y-axis isapproximately proportional to the signal induced in the monopole.

First, imagine the airplane to be flying straight and level, the nosepointed along the X-axis, toward the station. The segment OL representsthe monopole if it were oriented vertically. Now tilting it forward, byrotation through angle G about the Y-axis, results in the orientationOM. Since OM is in the X-Z plane, there is, as yet, no projection on theY-axis, and no signal induced in the monopole. Next, suppose theairplane banks, by rotation through angle B about the X-axis, resultingin the orientation ON. This is followed by a change in heading,corresponding to rotation through angle A about the Z-axis, whichresults in the final orientation of the monopole, along segment OP. Now,dropping a perpendicular onto the Y-axis, we have the projection 00,representing the induced signal. It can be shown by trigonometry thatthe length of O0 is given as follows:

OQ/OP cos G cos A sin B sin G sin A Note that for small bank and headingangles the first term in the above expression is roughly proportional tobank angle B, while the second term is roughly proportional to headingangle A; In applying the foregoing formula to an actual case, theforward tilt angle G should be carefully interpreted as an effectivevalue which includes due regard for axial dissymmetry of the airframeunder the antenna. With this in mind, the formula can be used, alongwith assumed or measured parameters of airspeed, cross-wind,course-width, indicator timelag, sensitivity factors, and initialconditions, to compute, with reasonable accuracy, the flight path to beexpected. The following explanation, however, is more graphic.

FIG. 5 is intended to provide an intuitive visualization of the flightpath followed by a pilot using the nullbank command meter of FIG. 3. Themeter pointer is centered when the polarization reference signal E2 isequal to the course displacement signal E1.

Beginning at position (a), assume a course deviation signal E1 equal to150 millivolts, corresponding to fullscale fly-left. The pilot,following the null-bank meter, simply rolls left to center. theneedleLTheprojection of the monopole on the horizon in the direction ofthe station is indicated by the dimension labelled E2 in FIG. 5, andcorresponds to the attitude voltage induced in the monopole. The roll tothe left stops at a bank angle such that E2 equals 150 millivolts. As aresult of the bank, the pilot naturally follows through with acoordinated left turn, leading toward position (b).

At position (b) there is an additional component of E2 due to theheading change. Also, E1 has decreased to, say, 105 mv. Thus, it isnecessary to roll out of the bank to keep E2 from becoming excessive.The cross course heading leads toward position (c).

At position (c), the course deviation signal El has been reduced to,say, 35 mv. With the existing crosscourse heading, it becomes necessaryto put in some right-bank in order to nullify some of the left-headingsignal and arrive at a resultant attitude signal E2 equal to 35 mv. As aresult of the right bank, a coordinated turn to the right naturallyfollows, leading toward position (d).

As position (d) the airplane is on-course, with El essentially zero; itis necessary to level the wings in order to bring E2 also to zero. Thuswe have seen the entire beam-bracketting process performedsemiautomatically, with no particular attention from the pilot otherthan to keep the wings more or less in conformity with the null-bankcommand meter. In fact if the meter voltage were used to-control aroll-axis autopilot servo, the beam-following could be quite automatic.

In the presence of a cross-wind, the system, as described, willstabilize with a course-deviation off-set. This off-set can be removedby any one of several methods which are customarily used in automaticflight control systems that employ gyros. One method would be to includea resistance-capacitance error integrator circuit as a part of thefilter-adder network 44. Such an addition, however, while resulting inimproved performance, does not really form a part of this inventionwhich is concerned primarily with the polarization reference antennasystem as a lowcost substitute for bank and heading gyroscopes.Similarly, if the effective forward tilt angle G is too small to producean adequate heading component of E2, then a bank integrator circuit canbe included in network 44 which will have the effect of augmenting oreven replacing the forward tilt heading component.

In passing, it should be noted that the polarization reference systemdoes have one characteristic, besides low-cost, which should beconsidered an advantage over gyros. That is, there is no need to worryabout erection to the vertical in the case of a bank gyro, or

setting a reference heading in the case of a directional gyro. These areboth built-in with the polarization reference system.

While the system has been described in connection with horizontallypolarized transmitting stations, it should be readily recognized thatthe method will work also with vertically polarized transmittingstations. In this case it is, of course, necessary to use a horizontallypolarized antenna on the airplane for obtaining the attitude signal, inconjunction with the normal vertically polarized antenna for receivingthe regular navigational signal.

We claim:

1. A polarization-referenced attitude system for use in flying the beamor course-line of a'linearly polarized radio-navigation facility incooperation with appropriate airborne radio equipment carried by anaircraft, including the usual navigation receiver with inputnavigational signal supplied by a first antenna means responsive in thepolarization plane of the radio signal when the aircraft is in levelflight; wherein the said attitude system comprises a second antennameans responsive in a plane perpendicular to the polarization plane ofthe radio signal when the aircraft is in level flight, modulation means,said second antenna means connected to said modulation means forproducing distinctive sidebands, and detecting means responsive to themagnitude and phase of said sidebands, producing thereby a signalindicative of departure from a reference attitude supplied by thepolarization plane.

2. An attitude system as defined in claim 1, with said first antennameans being responsive to horizontal polarization; wherein said secondantenna means comprises a conductor having some inclination from thevertical, but lying in the fore-and-aft vertical plane of symmetry ofthe airframe of said aircraft.

3. An attitude system as defined in claim 1, with an antenna systemcomprising two elements, having mirror symmetry about the airframecenter-plane, hybrid means connecting said elements in both sum anddifference modes, and with said first antenna means comprising one ofsaid modes; wherein said second antenna means comprises the other ofsaid modes.

4. An attitude system as defined in claim 1. wherein the said detectingmeans comprising a combining means arranged to add the said sidebands tothe regular navigational signal at the receiver input, a band-passfilter at the receiver output arranged to separate out the detectedmodulation associated with said distinc tive sidebands, a phase detectorderiving its phase reference from said modulation means, said filterfeeding said phase detector and producing thereby a rectified bi-polardirect current which is the said signal indicative of departure from areference attitude.

5. An attitude system as defined in claim 4 further comprising afiltering and adding circuit and wherein said bi-polar d.c. signalindicative of attitude is applied, along with the usual course-deviationsignal derived from the navigation receiver, to said filtering andadding circuit, a left-right bank command indicator responsive to theresulting combined signal, whereby a pilot, banking in response to saidindicator is caused to fly his aircraft smoothly to the center-line ofthe course defined by the navigation facility.

6. The method of producing aircraft pilot command signals by using apolarization-referenced attitude system entirely independent ofgyroscopes and gravity and the earths magnetic field comprising thesteps of:

deriving a standard type deviation indicator displacement signal from aconventional signal received from a standard linearly polarizedelectromagnetic field emanating from a conventional ground radionavigation facility;

developing a signal indicative of heading and attitude changes necessaryto maintain the aircraft on a level flight path directly toward thefacility by sensing deviation from the polarization plane of said field;

algebraically combining said derived and developed signals to obtain aresultant signal; and

indicating to the pilot by using said resultant signal as a commandsignal, the proper course, heading a nd attitude to be followed.

7. The method of claim 6 wherein the indicating step is carried out bydriving a null-bank command pointer from the resultant signal for visualuse by the pilot.

8. The method of claim 6 wherein the developed signal is initiallyreceived by an antenna fixed to the aircraft in predetermined tiltedposition relative to the plane of polarization of said electromagneticfield when the aircraft is headed at the facility in level flight.

9. The method of claim 8 further comprising modulating said initiallyreceived signal by audio band modulation; mixing the so-modulated signalwith the conventional signal; passing the so-mixed signals through aconventional navigation radio receiver; filtering from the audio outputof said receiver the audio modulation; and detecting the filtered audiomodulation against the original modulation to yield said developedsignal.

10. The method of claim 8 further comprising mixing the conventionalsignal received and the initially received signal to obtain sum anddifference signals; modulating the sum signal by audio band modulation;mixing the modulated sum signal and the difference signal; applying thelast mixed signals through a standard navigation receiver to derive saidstandard type deviation indicator displacement signal as the usualbi-polar D.C. displacement signal; filtering the audio band modulationfrom the receiver audio output; and detecting the filtered audio bandmodulation against the original modulation frequency as a reference todevelop said signal indicative of heading and attitude changes, also asa bipolar DC. signal for said algebraic combining with said usualbipolar D. C. displacement signal.

11. A polarization-referencing attitude method for enabling improvedflying the beam or course-line by aircraft of a linearly polarizednavigation facility broadcasting from earth, comprising the steps of:

deriving a conventional displacement type signal in the usual bi-polarD.C. form from an aircraft received signal from the facility afterpassing through the standard aircraft iiavigation receiver;

receiving a signal indicative of attitude and heading changes necessaryto correct to a course directly in line with the facility with theaircraft coming to level substantially at the line by picking up saidsignal at a linear location fixed in the center plane of the aircraftand, tilted through theplane of polarization when the aircraft is level;

modulating said received signal for identification with fixed frequencyaudio; mixing the so-modulated signal and the said aircraft receivedsignal from the facility;

passing the mixed signals through said standard receiver to derive saiddisplacement signal in DC. form;

filtering the audio output of the receiver to obtain the fixed frequencyaudio;

detectingthe filtered fixed frequency audio to produce a bi-polar DC.signal for attitude and heading changes;

algebraically adding the bi-polar D.C. signals to produce a resultantsignal; and driving a null-bank command pointer by the resultan signal.

1. A polarization-referenced attitude system for use in flying the beamor course-line of a linearly polarized radio-navigation facility incooperation with appropriate airborne radio equipment carried by anaircraft, including the usual navigation receiver with inputnavigational signal supplied by a first antenna means responsive in thepolarization plane of the radio signal when the aircraft is in levelflight; wherein the said attitude system comprises a second antennameans responsive in a plane perpendiculAr to the polarization plane ofthe radio signal when the aircraft is in level flight, modulation means,said second antenna means connected to said modulation means forproducing distinctive sidebands, and detecting means responsive to themagnitude and phase of said sidebands, producing thereby a signalindicative of departure from a reference attitude supplied by thepolarization plane.
 2. An attitude system as defined in claim 1, withsaid first antenna means being responsive to horizontal polarization;wherein said second antenna means comprises a conductor having someinclination from the vertical, but lying in the fore-and-aft verticalplane of symmetry of the airframe of said aircraft.
 3. An attitudesystem as defined in claim 1, with an antenna system comprising twoelements, having mirror symmetry about the airframe center-plane, hybridmeans connecting said elements in both sum and difference modes, andwith said first antenna means comprising one of said modes; wherein saidsecond antenna means comprises the other of said modes.
 4. An attitudesystem as defined in claim
 1. wherein the said detecting meanscomprising a combining means arranged to add the said sidebands to theregular navigational signal at the receiver input, a band-pass filter atthe receiver output arranged to separate out the detected modulationassociated with said distinctive sidebands, a phase detector derivingits phase reference from said modulation means, said filter feeding saidphase detector and producing thereby a rectified bi-polar direct currentwhich is the said signal indicative of departure from a referenceattitude.
 5. An attitude system as defined in claim 4 further comprisinga filtering and adding circuit and wherein said bi-polar d.c. signalindicative of attitude is applied, along with the usual course-deviationsignal derived from the navigation receiver, to said filtering andadding circuit, a left-right bank command indicator responsive to theresulting combined signal, whereby a pilot, banking in response to saidindicator is caused to fly his aircraft smoothly to the center-line ofthe course defined by the navigation facility.
 6. The method ofproducing aircraft pilot command signals by using apolarization-referenced attitude system entirely independent ofgyroscopes and gravity and the earth''s magnetic field comprising thesteps of: deriving a standard type deviation indicator displacementsignal from a conventional signal received from a standard linearlypolarized electromagnetic field emanating from a conventional groundradio navigation facility; developing a signal indicative of heading andattitude changes necessary to maintain the aircraft on a level flightpath directly toward the facility by sensing deviation from thepolarization plane of said field; algebraically combining said derivedand developed signals to obtain a resultant signal; and indicating tothe pilot by using said resultant signal as a command signal, the propercourse, heading and attitude to be followed.
 7. The method of claim 6wherein the indicating step is carried out by driving a null-bankcommand pointer from the resultant signal for visual use by the pilot.8. The method of claim 6 wherein the developed signal is initiallyreceived by an antenna fixed to the aircraft in predetermined tiltedposition relative to the plane of polarization of said electromagneticfield when the aircraft is headed at the facility in level flight. 9.The method of claim 8 further comprising modulating said initiallyreceived signal by audio band modulation; mixing the so-modulated signalwith the conventional signal; passing the so-mixed signals through aconventional navigation radio receiver; filtering from the audio outputof said receiver the audio modulation; and detecting the filtered audiomodulation against the original modulation to yield said developedsignal.
 10. The method of claim 8 further comprising mixing theconventional signal received and the initially received signal to obtainsum and difference signals; modulating the sum signal by audio bandmodulation; mixing the modulated sum signal and the difference signal;applying the last mixed signals through a standard navigation receiverto derive said standard type deviation indicator displacement signal asthe usual bi-polar D.C. displacement signal; filtering the audio bandmodulation from the receiver audio output; and detecting the filteredaudio band modulation against the original modulation frequency as areference to develop said signal indicative of heading and attitudechanges, also as a bipolar D.C. signal for said algebraic combining withsaid usual bipolar D. C. displacement signal.
 11. Apolarization-referencing attitude method for enabling improved flyingthe beam or course-line by aircraft of a linearly polarized navigationfacility broadcasting from earth, comprising the steps of: deriving aconventional displacement type signal in the usual bi-polar D.C. formfrom an aircraft received signal from the facility after passing throughthe standard aircraft navigation receiver; receiving a signal indicativeof attitude and heading changes necessary to correct to a coursedirectly in line with the facility with the aircraft coming to levelsubstantially at the line by picking up said signal at a linear locationfixed in the center plane of the aircraft and, tilted through the planeof polarization when the aircraft is level; modulating said receivedsignal for identification with fixed frequency audio; mixing theso-modulated signal and the said aircraft received signal from thefacility; passing the mixed signals through said standard receiver toderive said displacement signal in D.C. form; filtering the audio outputof the receiver to obtain the fixed frequency audio; detecting thefiltered fixed frequency audio to produce a bi-polar D.C. signal forattitude and heading changes; algebraically adding the bi-polar D.C.signals to produce a resultant signal; and driving a null-bank commandpointer by the resultant signal.